Big eater to understand about an opening for nutrition (6)
I believe the answer is:
gannet
'big eater' is the definition.
(I've seen this before)
'understand about an opening for nutrition' is the wordplay.
'understand' becomes 'get' (getting is a kind of understanding).
'about' is an insertion indicator.
'opening for' indicates taking the first letters.
The first letter of 'nutrition' is 'n'.
'an'+'n'='ann'
'get' going around 'ann' is 'GANNET'.
'to' acts as a link.
(Other definitions for gannet that I've seen before include "Act in accordance (with command)" , "greedy type" , "Large diving fish" , "Proverbially greedy seabird" , "Solan goose was this bird's former name" .)